This invention relates to a class of benzocyclobutene-terminated polyimide oligomers that are useful for high performance adhesive applications.
Because adhesives are lighter and more resistant to fatigue than mechanical fasteners, adhesive bonding of metallic and composite structures has become the preferred choice for military aircraft and aerospace applications. One concern about adhesively bonded joints is long-term stability. Metal-adhesive joints tend to degrade upon exposure to severe environments, particularly the high temperatures and humidities encountered in aircraft and aerospace applications.
Much effort has been directed toward the preparation of adhesives that exhibit high strength, good mechanical properties, and thermostability at temperatures above 200.degree. C. Epoxy adhesives are known to perform satisfactorily for over 15,000 hours at 150.degree. C., but only for short periods at 200.degree. C. For example, Adams teaches that epoxy phenolics, a mixture of epoxy and phenolic resins, degrade over a long period of time at 160.degree. C. (See R. D. Adams, "Adhesives for High-Temperature Applications", European Conf. on High Temperature Materials in Engineering Metals, Ceramics, Plastics, Apr. 27-28, 1989.)
Polyimides have also been targeted for high performance adhesive applications, primarily for their high thermostability. Unfortunately, they tend to be intractable in their fully imidized form, making processing difficult. To address this processing problem, flexibilizing groups such as fluoroalkylene units (J. P. Critchley et al, J. Poly Science, vol. 10, page 1809, (1972)), or bulky side groups (Harris et al. in "Polyimides: Synthesis, Characterization and Applications", K. L. Mittal, ed. Plenum, New York, volume 1, page 3, (1984)) or asymmetric catenates (Burks et al. in "Polyimides: Synthesis, Characterization and Applications", supra, page 119) have been incorporated into the polymer backbone to achieve tractable, processable polyimides. However, this improvement in processability is offset by the undesirable decrease in solvent resistance as well as glass transition temperature. Polyimides also require a curing step, during which volatiles are released. This is particularly a problem when curing large-area metal-to-metal surfaces, where volatiles can be trapped and cause voids in the bonded joint. (See Millard in "Adhesive Bonding of Al Alloys", E. W. Thrall and R. W. Shannon, eds., volume 8, Marcel Dekker, Inc., New York, pages 129 and 133, 1985.)
One way to solve the problem of volatile release upon curing is to endcap already cyclized imides with benzocyclobutene (BCB) prior to a curing step. Then, upon curing, the BCB polymerizes without releasing volatiles. Kirchhoff (U.S. Pat. No. 4,540,763, incorporated herein by reference) discloses bisbenzocyclobutenes and polymers derived therefrom. The bisbenzocyclobutene moieties are connected by a direct bond or through a bridging group, such as a cyclic imido group. Tan and Arnold (U.S. Pat. No. 4,711,964, incorporated herein by reference) discloses the synthesis of bisbenzocyclobutene imide compounds, the polymers of which form high temperature-resistant thermoset resins that are useful in composite materials in advanced aircraft and aerospace vehicles.
What remains elusive is a BCB endcapped imide oligomer which 1) has a glass transition temperature, T.sub.g, that is lower than the temperature at which polymerization of the BCB moieties proceeds rapidly; 2) forms a polymer that is thermally stable above 200.degree. C.; 3) does not release volatiles upon curing and; 4) has adhesive properties that are acceptable for high performance applications.